(1) Field of the Invention
The present invention relates to an electrophotographic process using an organic photoconductive photosensitive layer. More particularly, the present invention relates to an electrophotographic process in which the surface potential of a photosensitive layer is always stable irrespectively of the environmental change or the copying cycle number and a stable image is always formed.
(2) Description of the Prior Art
In a commercial electrophotographic copying operation, there is ordinarily adopted a system in which after charging a photosensitive layer by direct current corona discharge, operations of imagewise light exposure, toner development, toner image transfer and cleaning are repeated the necessary number of times.
The relation between the injection current value from a corona charger and the surface potential of a photosensitive layer at the charging step is ordinarily such that under certain conditions, the surface potential is monotonously increased with increase of the injection current. However, the injection current of the corona charger is greatly changed by environmental changes such as a change of the power source voltage and a change of the distance between the charger and the photosensitive layer, and by this change of the injection current, the surface potential of the photosensitive layer is changed. This tendency is conspicuous in an organic photoconductive photosensitive layer, in which injection of charges is not so easy as in a selenium type photosensitive plate, and the image quality is changed by the change of the surface potential owing to environmental changes in the organic photoconductive photosensitive layer.
When the copying operation is carried out repeatedly many times on an organic photoconductive photosensitive layer, the image density of prints is reduced with increase of the cycle number. Although the reason of this undesirable phenomenon is indefinite, it is considered that a carrier having a relatively long life is formed in the organic photoconductive photosensitive layer and injection of charges on the surface is rendered difficult by the space charge-controlling action of this carrier.